A liquid crystal display comprises a liquid crystal cell, a polarizing sheet and an optical compensatory sheet (phase retarder) provided between the liquid crystal cell and the polarizing sheet.
The liquid crystal cell comprises a pair of substrates, a rod-like liquid crystal compound and an electrode layer. The rod-like liquid crystal compound is provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal compound. Each of the substrates has an orientation layer (both-sided orientation layers), which has a function of aligning the rod-like liquid crystal compound. The orientation layer of the liquid crystal cell is usually prepared by forming a polymer (e.g., polyimide, polyvinyl alcohol) membrane on the substrate, and rubbing the membrane with a cloth uniformly.
The optical compensatory sheet is a birefringent film having a function of removing color from an image displayed in liquid crystal. A stretched birefringent resin film has usually been used as the optical compensatory sheet.
It has been proposed to use an optically anisotropic element as the optical compensatory sheet in place of the stretched birefringent film. The optically anisotropic element is formed by fixing an aligned liquid crystal compound. Liquid crystal compounds have various alignment forms. Therefore, an optically anisotropic element obtained by fixing an alignment form can have a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film.
Japanese Patent Provisional Publication No. 3(1991)-87720 discloses an optical compensatory sheet of a liquid crystal display. The optical compensatory sheet is made of an aligned membrane of a twisted nematic phase. The optical compensatory sheet uses a rod-like liquid crystal compound, which is similar to a liquid crystal compound used in a liquid crystal cell. Japanese Patent Provisional Publication No. 5(1993)-215921 discloses an optical compensatory sheet comprising a rod-like compound provided between a pair of substrates, which have been subjected to an alignment treatment. The rod-like compound shows a liquid crystal characteristic after hardening the compound.
However, the requirements of the optical compensatory sheet about the characteristics of the liquid crystal compound and the orientation layer are completely different from the requirements of the liquid crystal cell.
For example, the optical compensatory sheet requires an effect of enlarging viewing angle. However, the effect of the rod-like liquid crystal compound used in the liquid crystal cell is insufficient. Further, the optical compensatory sheet uses the liquid crystal compound fixed as an aligned state. However, the alignment of the rod-like liquid crystal compound is not stable. Further, the rod-like liquid crystal compound used in the liquid crystal cell is not sufficiently aligned on one orientation layer (single-sided orientation layer) provided on a substrate (support). Since the liquid crystal cell has a pair of substrates between which the rod-like liquid crystal compound is provided, an orientation layer can be provided each of the substrates (both-sided orientation layers). If an optically compensatory sheet has the both-sided orientation layers similarly to the liquid crystal cell, as is described in Japanese Patent Provisional Publication No. 5(1993)-215921, the total weight of the liquid crystal display device is greatly increased.
For the reasons mentioned above, the liquid crystal compound and the orientation layer for the optical compensatory sheet have been researched and developed independently to the liquid crystal compound and the orientation layer for the liquid crystal cell.
With respect to the liquid crystal compound for the optical compensatory sheet, it has recently been proposed to use a discotic liquid crystal compound in place of a rod-like liquid crystal compound. The viewing angle of the liquid crystal display can greatly be enlarged by using the discotic liquid crystal compound in the optical compensatory sheet. Further, the alignment of the discotic liquid crystal compound is stable, compared with the rod-like discotic liquid crystal compound. Japanese Patent Provisional Publication No. 8(1996)-27284 proposes further stabilizing the alignment of the discotic liquid crystal compound by introducing a polymerizable group to the compound and polymerizing the compound after the alignment.
With respect to the orientation layer for the optical compensatory sheet, a single-sided orientation layer has been proposed to control the alignment of the discotic liquid crystal compound. Japanese Patent Provisional Publication No. 7(1995)-146409 discloses an obliquely evaporated silicon oxide (SiO) membrane, which can be used as a single-sided orientation layer in place of a conventional orientation layer obtained by a rubbing method. Japanese Patent Provisional Publication No. 7(1995)-281028 discloses a rubbed polyimide membrane used as the single-sided orientation layer.